In the papermaking industry, substituting inorganic filler for wood fiber in paper and paperboard is advantageous because the inorganic filler is generally less expensive than wood fiber and the substitution lowers costs. Precipitated calcium carbonate is commonly used as a filler in the industry. Although inorganic fillers decrease the total cost of papermaking, increasing concentrations can reduce the overall bulk, strength, and stiffness of the paper—all of which are important end use performance properties.
This decrease in strength and stiffness in the final paper product is a result of the structure of the wood pulp and inorganic filler. During the papermaking process, the long wood pulp fibers become entangled, thus creating a strong web of fiber. The inorganic filler does not have these long fiber chains, so increasing the inorganic filler content can weaken the fiber web in the finished product. In addition, as the inorganic filler content increases, the never-dried strength of the wet web exiting the press section of a paper machine decreases. This strength decrease affects machine runnability and may force the paper machine to run at lower yields because of a lower thru-put or higher downtime because of web breaks in the wet web.
Although the prior art teaches treatments, as part of the papermaking process, for increasing the retention of fine inorganic fillers in the final paper or paperboard product, the prior art does not disclose methods to increase the inorganic filler content of paper while simultaneously maintaining the weight, strength, and runnability of the end product.
For example, dry strength resins are known in the prior art and can increase the strength of the finished paper product when mixed into the initial paper pulp slurry (also called a paper furnish). Amphoteric, water-soluble dry strength resins are known in the prior art. Amphoteric resins are typically made by reacting acrylamide with cationic and anionic monomers (for example, diallyldimethylammonium chloride (“DADMAC”) and acrylic acid) in a free radical copolymerization reaction. These resins are generally limited to 10-15 mol % of each ionic component (20-30 mol % charged polymer total). If the ionic polymer concentration is higher, the solution becomes unstable.
Additionally, separate anionic and cationic polymeric dry strength resins are also known in the prior art. Typically, these resins will be added sequentially—i.e. all the resin of one charge is added, then all the resin of the opposite charge is added. When anionic and cationic resins are added as separate resins, the anionic resin is typically an acrylamide/acrylic acid copolymer. The cationic typically contains either DADMAC, acryloylethyltrimethylammonium chloride (“AETAC”), or a hydrolyzed form of vinyformamide.
For example, the inorganic filler content of paper may be increased by treating the pulp slurry and inorganic filler separately with a charged polymer, then treating the filler with an oppositely charged ionic, and mixing the treated filler and pulp slurry together. Alternatively, one may treat only the inorganic filler with a charged polymer, and then combine the treated filler with the pulp slurry for processing into paper.
Another method to maintain paper bulk as the inorganic filler content of paper is increased is to increase the average inorganic filler particle size. An increase in filler concentration and/or filler particle size can lead to additional abrasion on the paper slurry processing surfaces. This abrasiveness generally manifests itself as additional wear on the wet end of the paper making process, especially on the paper forming fabrics and static drainage elements. Additionally, the increased wear on these parts, slitter knives, and other surfaces may degrade the quality of the final paper product and increase maintenance and servicing costs for the equipment. Previous attempts to mitigate these problems have included addition of surfactants and TEFLON (polytetrafluoroethylene) to the paper slurry.